This would give me the Invaders to play around with - Cap, Namor and Alan. Wakanda, Atlantis (Marvel), Theymiscra.

Cap vs Batman fights. Namor vs Wonder Woman.

Red Skull, possibly the Oans, Batman's rogue gallery.

Which fight would you like to set up?

if I'm looking for ROI, I might focus on how much I have to pay to buy...Superman, Batman, Wonder Woman, Spider Man, Hulk, Wolverine, provided the price is right.

If I'm looking to build something that could be cool, with characters I like and think I could make some interesting stories with, I might pick more obscure characters, or maybe some villains....some ideas as follows:

Lets go with Venom, Sabretooth,

DC:
Mongul- semi recognizable, especially to DC fans, and getting the Warworlders and Warworld allows a great villain and backdrop for some great stories

Dove 2: As depicted near the end of the Kessel run, one of my ATF characters...not going to pull many people through name recognition, but thats why I'm targeting villains

Sinestro: If I can get him reasonably priced, as he carries a lot of cache in the comics, but not so much in the non comic world, so should be cheaper than getting someone like Darkseid or Doomsday. Interstellar background provides a reason why he may make it to the new universe where he won't be plagued by Green Lanterns...

I also considered Orion, Blue Devil, and Massacre because I really like them, but only picked one of the characters I really liked. I also considered Speed McGee, but he carries no cache...

Marvel:
Venom or Sabretooth: both are likely to be expensive, but both also carry substantial recognition due to their affiliation with major heroes. Either would make a great nemesis for someone like Dove.